Effects of repetitive stimulation and state of arousal on cochlear potentials

Abstract

Click-evoked responses were recorded from the round window (RW) in unanesthetized cats. In a preliminary study the effects of repetitive stimulation were found to be confounded with changes due to variations in arousal state. Two procedures were employed to evaluate the effects of such stimulation independently of arousal. In the first experiment RW responses were obtained during different arousal states prior to and following repetitive 60-db click stimulation. Averaging techniques were employed to separate the cochlear microphonic (CM) and auditory nerve action potential (AP) components of these responses. Pre- and post-stimulation comparisons for each arousal state gave no evidence of habituation decrements under these controlled conditions. The general pattern of amplitude variation with arousal was in accordance with the hypothesized dependence of such changes on nonreflex middle ear muscle activity. Certain features of the results, however, suggested that a second factor, masking by noise generated by the animal's own movements, was also involved. In the second study there was no evidence of response decrements to repetitive 45-db click stimulation in animals in which the middle ear muscles had been unilaterally sectioned. These results are contrary to theories of peripheral “gating” of auditory input.